The increasing use of information processing devices such as word processors, high speed photocopiers, and the like has resulted in the emergence of various types of printers to output the information. For applications in which the emphasis is on quietness, high speed and high print quality, the most extensively used printers are electrophotographic printers. Laser printers and liquid-crystal printers are two examples of such printers, but in addition there is the LED printer, which uses a print head comprised of arrays of LED elements. In the LED printer, printing involves using image data signals applied to LED driver integrated circuits (ICs) to drive the print head LED array elements and form an electrostatic latent image on a photosensitive medium, and the image is then developed and transferred to paper.
The LED element is a semiconductor device that emits radiation in the optical spectrum (i.e., infrared through ultra violet) in response to an applied forward bias voltage from an external power source, in this case, the driver IC. An LED has a p-n junction provided by two extrinsic gallium arsenide alloy-based semiconductors. When forward biased by an external power source, such a diode emits optical radiation or light which can discharge a charged photosensitive medium. LEDs are attractive light sources because they are easy to form, inexpensive to make, highly efficient, and reliable.
High-density, linear LED arrays can be fabricated in monolithic arrays and used as light sources in conjunction with driver ICs and lens system of a print head. In printing applications, light from the diodes of an array forms a line of light images at corresponding pixels of an image zone. A photosensitive medium is positioned (a line at a time) at the image zone to be exposed by these light images. With existing technology, it is possible to fabricate an array of light-emitting diodes on a single gallium arsenide chip. Discrete regions of the array constitute a picture element region (pixel) of the array. Each array pixel includes a single LED region which can illuminate a particular image zone pixel. Binary and grey scale exposures of image zone pixels may be produced by selectively modulating the current level through or the on time of the LED elements for either fixed (binary) or variable (grey scale) printing.
Current state of the art in the manufacture of LED based optical printhead systems is to obtain as flat a light profile as possible in the intensity of the light emitted across the light-emitting surface of the active region of each individual LED element of the LED arrays on a printhead. Uniformity of the light intensity from LED region to LED region of the printhead is critical in such printing applications because the photosensitive medium is capable of resolving fine variations in light intensity and reproduce the variations as streaks or rasterbands extending down the moving receiver or print media.
In order to print a line of print across standard width paper with sufficient resolution, a linear LED printhead is constructed of a series of LED arrays aligned end-to-end, wherein each LED array typically comprises 64, 96, 128 or 196 LED elements. The LED elements on each LED array are aligned in a row and each LED element occupies a certain amount of space on the array. The physical size and spacing of the LED elements, together with an appropriately sized SELFOC lens array for focusing light from the LEDs onto the photosensitive medium, provides standard resolution levels for printheads such as 200, 240, 300, 400, 480 and 600 dots per inch (DPI) or 7.87, 9.45, 11.81, 15.75,18.90 and 23.62 dots per mm (DPMM).
In such optical printer printheads, the LED array and LED driver circuit ICs may be mounted on a metallic "tile" or a ceramic substrate and electrically connected by wire bonding. FIG. 1 shows a known method of mounting an optical printer LED array and LED driver ICs where the odd and even numbered sets of LED elements are connected respectively to odd and even driver circuit ICs 12 and 14. An optical printhead LED array 10 and the driver ICs 12 and 14 are arranged side-by side in straight lines on the tile 16 or other substrate and bonded thereto with an adhesive such as silver epoxy resin. A series of LED arrays and a series of driver ICs are each assembled end-to-end on the substrate to form an elongated LED printhead having a gap 19 between each LED array that maintains uniform spacing between adjacent LEDs at the ends of butting arrays. Examples of such printheads are well known, for example, see U.S. Pat. No. 5,317,344.
The LED arrays 10, 10' each comprise a fixed number, e.g. 128, individual LED elements 18, for 400 DPI or 15.75 (DPMM) resolution, wherein the 64 odd and even LED elements each have an associated bonding pad arranged in pad array rows 26 and 28 extending alongside either side of the row of LEDs. Rows 22 and 24 of driver IC bonding pads are respectively lined up adjacent to the LED pad rows 26 and 28 and formed on the driver ICs. Pads in adjacent pad rows are connected by wire bonding using aluminum or gold wires 30 several tens of micrometers in diameter to thereby enable the LED elements to be driven by the driver circuit ICs 12 and 14.
Surface metal trace conductors 21 extend from the LED bonding pads and terminate in electrodes 21' overlying an active, light-emitting region 18 of each LED element. The electrodes 21' are opaque and block the emission of light from the element. Typically, the electrodes 21' are narrow bands or finger-like traces of metallization extending across the center of the active region 18 as shown in FIG. 1. The width of the electrodes is typically selected in relation to the width of the light-emitting region 18 of each element to take into account the current spreading resistance which causes light output to fall off as one moves farther from the electrode 21' and toward the lateral edges of the region 18. When individual LEDs are formed in an array as shown in FIG. 1, it is necessary to maintain a gap 23 between individual diffusion areas of adjacent LED elements to keep the devices from shorting together. These effects and requirements cause a repeating pattern of reduced light intensity extending lengthwise along the LED array which are measurable as a varying intensity light profile. For high resolution printing systems, the varying intensity light profile, if imaged by the lens system on the moving photosensitive medium, can be seen in the successive line images printed as streaks or rasterbands.
LED arrays of the type shown in FIG. 1 employed in photocopiers are typically coupled with a SELFOC lens array (trademark of Nippon Sheet Glass) or other optical imaging systems. Examples of mounts for assembling the SELFOC lens array to an array of LEDs that have integrated circuit drivers connected thereto for providing current to the LEDs is shown in U.S. Pat. No. 4,728,981, the contents of which are incorporated herein by reference. The particular mount is not critical and others are also known in the prior art. In FIG. 12, there is shown a prior art LED array 200 SELFOC lens array 205 and having recording fibers 210 and recording member or medium 220 are illustrated to show their general relationship. The light emitted by each element, located generally at an object plane of the lens array, is focused onto the electrostatically charged photosensitive medium or other types of media such as photographic film, etc. so that image pixels may be generated from the selective energization of the LED elements following a number of known LED printing conventions. The lens array is composed of a number of columnar lens elements which are similar to short light pipes having a diameter that is greater than the width of the high resolution LED elements. The lens array thus focuses light from more than one LED element onto an exposure line on the charged photosensitive medium that is located generally at an image plane of the lens array. The lens array exhibits a specific Modulation Transfer Function (MTF) and Line Spread Function (LSF) that roughly relates to the sharpness of focus of light entering the lens element at one end and exiting onto an image pixel space in a line on the photosensitive medium.
The SELFOC lens is a gradient index lens which exhibits blurring of the image as characterized by the lens' MTF curve as described, for example, by Carellas and Fantone, "Lens Testing: The Measurement of MTF", PHOTONICS SPECTRA, pp. 133-138, September 1989. An MTF curve for a particular lens is a mapping of contrast, indicated in percent, against spatial frequency, indicated in line pairs per millimeter (lp/mm). The MTF of a lens is typically tested at the center of the lens' field of coverage or at other points displaced from the center and at various apertures, if the aperture is variable. The SELFOC lenses have fixed apertures, and testing is conducted on axis through the center of the lens array with LEDs centered.
The effect of this blurring is to cause the light profile of the individual LED element after focusing by the SELFOC lens to be further altered. Since it is the light profile of an energized LED element after lensing, i.e. passing through the lens, which exposes image pixels on the photosensitive medium, the lens' characteristics affect image quality.
Moreover, when a series of LED arrays 10, 10' and associated driver circuit ICs 12, 12' and 14, 14' are assembled end-to-end to form a linear LED print head array 11 of sufficient length to image lines on a photosensitive medium to print full sized text or images, imperfect assembly of the LED arrays may result in further light profile changes.
In U.S. Pat. No. 4,956,684, the light profile emitted by the array of LED elements in a single IC and abutted ICs making up a linear IC array is modified to decrease the attenuation at the edges of the adjoining LED regions by applying a diffractive coating overlying the edge portions of adjacent regions. The amplitude extremes in the light profile before and after lensing are said to be diminished. Other approaches to smoothing out the light profile of the LED array are summarized in the Background of the Invention of the '684 patent. Despite these approaches taken to reduce the light profile variations, a problem remains in image quality due to light profile intensity variations contributed by the size and spacing requirements of the LED regions, the LED energizing electrodes and the assembly of separate LED arrays into a linear LED print head.